1. Field of the Invention
This invention relates to vehicle pneumatic brake systems such as are employed on large trucks and the like. More particularly, it relates to an improved brake system for maintaining vehicle stability by preventing under substantially all operating conditions wheel lock-up upon brake application and braking induced rotational vehicle movements, which may otherwise produce skidding and loss of lateral vehicle control.
2. Description of the Prior Art
Automatic control systems for vehicle pneumatic brakes have been designed with "anti-lock" or "anti-skid" features which, when pneumatic supply pressures are within certain limits, insure vehicle stability upon brake application. Some such prior art systems are shown in U.S. Pat. Nos. 2,937,052, 3,059,975, 3,439,958, 3,920,045.
In particular, a system is shown in U.S. Pat. No. 3,920,045, wherein an anti-lock, pressure modulating valve is combined with a modified relay valve. While supply pressure is within preset bounds (i.e., "normal"), the combined anti-lock/relay valve supplies braking pressure suitable for decelerating the vehicle and avoiding wheel lock. When, however, "normal" conditions do not obtain, i.e., supply pressure is less than the preset bounds, the anti-lock, pressure modulating valve is bypassed and braking pressure is supplied to the brakes by the modified relay valve. While this system has performed well, it would be desirable to provide a system wherein even under abnormal conditions the anti-lock, pressure modulating valve is maintained as a part of the braking control apparatus, thereby providing braking pressure to the brakes suitable for decelerating the vehicle while avoiding wheel lock-up.